J-W Nah et al., ‘A Study on Coining Processes of Solder Bumps on Organic Substrates,’ IEEE Transactions on electronics packaging manufacturing, Vol. 26, No. 2, April 2003, p. 166, shows a current high volume manufacturing method of forming solder bumps on laminates. The solder paste stencil printing method has been used for forming solder bumps on organic substrates for a long time. As seen in FIGS. 1A and 1B, an organic substrate 1002 is provided with a plurality of pads 1006 and a solder resist layer 1004. A stencil mask 1008 is aligned therewith. As in FIG. 1C, solder paste 1012 is dispensed through the holes in mask 1008 using a squeeze knife or blade 1010. The mask is removed as in FIG. 1D leaving upstanding solder paste regions 1014. FIG. 1E depicts reflow wherein regions 1014 become rounded bumps 1016 with flux residue 1018 thereon. The same is cleaned away in FIG. 1F leaving solder balls 1016 with a projecting height above the resist shown at 1020.
FIGS. 1A-1F thus illustrate a prior-art technique for forming solder bumps on substrates. Solder paste is stencil printed through a mask aligned to the substrate solder resist (SR) openings. The mask enables deposition of solder paste material that stands above the SR after mask removal.
However, commercial applications of the solder paste stencil printing method are limited to substrates with pitches greater than or equal to 150 microns because the high volume percentage of flux (around 50 volume %) in the solder paste hinders formation of high volume solder bumps without solder bridging.
U.S. Pat. No. 6,213,386 of Inoue et al., entitled ‘Method of forming bumps,’ discloses wherein solder balls and a tool having a large number of through-holes are used, and under the condition that the through-holes of the tool are aligned with the pads of the semiconductor device, the solder balls are charged into the through-holes, pressed to be fixed on the pads, and then reflowed to form bumps.
FIGS. 2A-2H illustrate a prior-art pre-formed solder ball mounting method for fine pitch applications under 150 microns pitch. Elements similar to those in FIGS. 1A-1F have received the same reference character. As seen in FIG. 2A, an organic substrate 1002 is provided with a plurality of pads 1006 and a solder resist layer 1004. A mask for flux 2020 is aligned therewith. As in FIG. 2B, tacky flux 2002 is applied, and as in FIG. 2C, mask 2020 with adherent flux 2002 is removed, leaving flux portions 2004 on pads 1006. A mask 2006 for solder ball dispensing is aligned with the pads, as in FIG. 2D, and in FIG. 2E, preformed solder balls 2008 are located in openings in the mask 2006, in contact with flux portions 2004, which retain them when the mask 2006 is removed as in FIG. 2F. FIGS. 2G and 2H depict reflow and flux cleansing and are similar to FIGS. 1E and 1F.
FIGS. 2A-2H thus illustrate another prior-art technique for forming solder bumps on substrates; the so-called micro ball mounting method wherein two masks are used: one to dispense tacky flux and the other to place preformed solder balls on the pads of the substrate. The tacky flux makes the balls stick to the bottom of the SR opening.
Solder balls 2008 may be expensive and flux application and solder ball dispensing may be somewhat complex. Furthermore, this type of technology needs additional steps to form uniform height solder bumps on different size pads.